It is well known that the load of consumables such as propellant must be maximized when spacecraft are launched. Similarly, the amount of consumables used for each maneuver should be minimized, to achieve the greatest possible on-orbit or on-station lifetime.
Reaction wheel and magnetic interaction schemes have been used to aid in attitude control when the spacecraft is on-station to reduce the need to use thrusters, to aid in extending the life of a spacecraft by reducing the consumption of consumables. Also, when on-station, the spacecraft may from time to time require operation or burn of one or more thrusters for orbital maneuvers, such as imparting a velocity change, as occurs, for example, for pre-operational orbit circularization, or for maintenance of north-south station (stationkeeping). Disturbance torques are generated during thruster burn due to offsets between the actual center of mass of the spacecraft and the thrust axis of the thruster. These offsets may be due to unavoidable errors in determining the center of mass and in aligning the thrust axis. Also, when the propellant flow rate is high, as when a thruster is used to leave a transfer orbit, the actual center of mass may move during thruster burn due to uneven draw of propellant from the tanks. Other causes of torques include engine exhaust plume impingement on various spacecraft surfaces. In order to maintain the spacecraft attitude during an orbital maneuver, it may be necessary to burn or operate attitude control thrusters, to counter unwanted torques which accompany the firing of a velocity-change-inducing thruster. Thus, more propellant may be used when a velocity change is commanded than is required for the velocity change itself, with the additional propellant being used by attitude control thrusters to maintain attitude during the maneuver. An improved attitude control method is desired.